RepoIndex: test all permutations of the pretty setting

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
RepoIndex: clarify behaviour of pretty setting

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
RepoIndex: make code reflect clarification table

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
RepoIndex: add 'compressed' option

We can now control pretty printing and compression individually,
while preserving the original behaviour.

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>